Conventional electronic control units as examples of computer circuits for vehicles are provided with a microcomputer operative to execute various tasks for controlling a target, and a power supply circuit. The power supply circuit is designed to step down a voltage supplied from a battery to predetermined constant voltages, and to feed the stepped down voltages to the microcomputer as operating voltages (power supply voltages).
An example of such electronic control units is disclosed in Japanese Examined Patent Publication No. 3,217,730. In the Examined Patent Publication, an electronic control unit is configured to interrupt the supply of power supply voltages to a microcomputer from a power supply circuit when the microcomputer goes into a sleep mode, thereby reducing consumption current therein.
Specifically, the electronic control unit disclosed in the Examined Patent Publication is provided with a pair of CAN buses, and a semiconductor circuit disposed between the paired CAN buses and a bus protocol module installed in the microcomputer; this semiconductor circuit is operative to control communications between the microcomputer and other devices through the paired CAN buses.
When the electronic control unit shifts into a sleep mode by the state signal of the microcomputer, the semiconductor circuit outputs an interruption signal to a voltage regulator as the power supply circuit; this interruption signal directs the voltage regulator to interrupt the supply of the operating voltages to the microcomputer. This results in that no operating voltages are supplied to the microcomputer from the voltage regulator.
In addition, when receiving an exteriorly sent wakeup signal through the paired CAN buses or other signal lines, the semiconductor circuit outputs, to the voltage regulator, an activate signal directing the voltage regulator to activate the microcomputer. Thus, the voltage regulator supplies the operating voltages to the microcomputer, thereby activating the microcomputer.
Note that the interruption signal and the activate signal are configured to be output from the semiconductor circuit to the voltage regulator through a single signal line. For example, when a signal with a low level is sent from the semiconductor circuit to the voltage regulator through the single signal line, the signal serves as the interruption signal. In addition, when a signal with a high level is sent from the semiconductor circuit to the voltage regulator through the single signal line, the signal serves as the activate signal.
In the disclosed Patent Publication, the state signal of the microcomputer output therefrom specifically means a signal output from the microcomputer to request the semiconductor circuit to interrupt the supply of the operating voltages to the microcomputer when the microcomputer determines that it is allowed to put itself to “sleep”. Specifically, the electronic control unit disclosed in the Examined Patent Publication is configured such that the semiconductor circuit recognizes the state signal output from the microcomputer and controls the power supply circuit to interrupt the supply of the operating voltages to the microcomputer in response to the recognition.
The configuration of the electronic control unit disclosed in the Examined Patent Publication may however interrupt the supply of the operating voltages to the microcomputer to put the electronic control unit out of operation in the case of only a semiconductor-circuit failure.
A semiconductor-circuit failure also may cause a cessation of the microcomputer activation, in other words, the electronic control unit activation.
Moreover, in the Examined Patent Publication, power-supply control functions for “sleep” and “wakeup” of the microcomputer have been collectively installed as circuit elements in the semiconductor circuit for communication control between the microcomputer and other devices through the paired CAN buses. This may cause waste of circuit resources when the electronic control unit needs to be provided with the number of paired CAN buses for the following reason:
Specifically, establishment of the number of paired CAN buses requires the corresponding number of the semiconductor circuits provided in the electronic control unit. Because any one of the semiconductor circuits probably handles the power-supply control functions, the circuit elements offering the power-supply control functions installed in the remaining semiconductor circuits may be wasted, causing the cost of the electronic control unit to increase. The number of the semiconductor circuits provided in the electronic control unit may require a specific configuration for preventing between the respective power-supply controls of the plurality of semiconductor circuits.